Most peptide and protein drugs are short-lived and have often a short circulatory half-life in vivo. This is particularly valid for nonglycosylated proteins of a molecular mass less than 50 kDa. The short lifetime of proteins in vivo is attributed to several factors, including glomerular filtration in the kidney and proteolysis. Considering that peptide and protein drugs are not absorbed orally, prolonged maintenance of therapeutically active drugs in the circulation is a desirable feature of obvious clinical importance. Proteins with molecular masses above ˜60 kDa largely avoid glomerular filtration and are not, for the main part, filtered in the kidney. Therefore they remain in the circulation longer than smaller proteins.
An attractive strategy for improving clinical properties of small protein drugs has come to be known as PEGylation (or pegylation, as used hereinafter). By this strategy several hydrophilic chains of polyethylene glycol (PEG) are covalently linked to the protein in order to increase its effective molecular mass. Important clinical advantages are gained by pegylation. For example, life-time in vivo can be prolonged in some instances from minutes to hours, owing to the steric interference that protects conjugates from proteolysis in vivo and the increase in molecular mass, which precludes filtration by the kidney. Protein pegylation also decreases immunogenicity, presumably by protecting conjugates from being recognized as foreign antigens by the immune system.
In spite of the profound advantages often gained by pegylating therapeutic proteins, this technology suffers from a principal drawback. On the one hand, covalently attaching PEG chains to proteins prolongs their lifetime in vivo, protecting the conjugates from proteolysis and shielding them from the immune system. On the other hand, the steric interference of the PEG chains often leads to a drastic loss or even abolish the biological and the pharmacological potencies of the proteins in the conjugates (Fuertges and Abuchowski, 1990; Katre, 1993; Bailon and Berthold, 1998; Nucci et al., 1991; Delgado et al., 1992; Fung et al., 1997; Reddy, 2000; Veronese, 2001). In principle, this deficiency can be overcome by introducing the PEG chains via a chemical bond that is sensitive to hydrolysis, or can be cleaved enzymatically by serum proteases or esterases. Clearly, a consistent rate of hydrolysis is crucial. A prerequisite condition is therefore that the hydrolysis of the PEG chains from the conjugate is to take place at a slow rate, and in a homogenous fashion in vivo.
It would be highly desirable to design PEG derivatives of proteins or peptides or small drug molecules from which PEG can be released by hydrolysis. An appropriate reversible PEG conjugate would have to be hydrolyzed slowly and spontaneously in physiological conditions and would permit time-dependent reactivation of inactive pegylated proteins and peptides.
Several methods for reversible pegylation were proposed (Greenwald et al., 1999, 2000; Lee et al., 2001; Garman and Kalindjian, 1987; Zalipsky et al., 1999). They suffer, however, from major potential drawbacks. For example, reliance on enzymatic detachment as a rate-determining step (Greenwald et al., 1999, 2000; Lee et al., 2001) of PEGs from conjugates by serum proteases and/or esterases might not yield desirable pharmacokinetic profiles in situ. Moreover, it is dependent on enzymes availability. Disulfide-bonded conjugate is not to be cleaved in the non-reducing environment of the body fluids (Zalipsky et al., 1999). A reversibly pegylated conjugate which still retain an active moiety capable of reacting with free SH functions may result in complex undesired cross-linking (Garman and Kalindjian, 1987). It would be very desirable to design a version of reversible pegylation that would overcome these deficiencies.
International PCT Publication No. WO 98/05361 of the present applicants describes a novel conceptual approach for prolonging the half-life of drugs by derivatizing a drug having at least one free amino, carboxyl, hydroxyl and/or mercapto groups with a moiety that is highly sensitive to bases and is removable under mild basic conditions. The prodrug obtained is inactive but undergoes transformation into the active drug under physiological conditions in the body. Examples of said moieties are the radicals 9-fluorenylmethoxycarbonyl (Fmoc) and 2-sulfo-9-fluorenylmethoxycarbonyl (FMS). According to this concept, Fmoc and FMS derivatives of peptidic drugs such as insulin and human growth hormone as well as of non-peptidic drugs such as propanolol, cephalexin and piperacillin have been described in said WO 98/05361. Later on, FMS derivatives of cytokines have been disclosed in WO 02/36067, and FMS derivatives of enkephalin, doxorubicin, amphotericin B, gentamicin and gonadotropin releasing hormone (GnRH) were disclosed in WO 02/7859.
U.S. Pat. No. 6,433,135 discloses a pegylated derivative of an analogue of luteinizing hormone releasing hormone (LHRH or GnRH) in which the PEG moiety is covalently bound to a serine residue of said LHRH analogue. In the process of preparation of said PEG-LHRH analogue by solid phase peptide synthesis, a pegylated serine residue such as Fmoc-Ser(PEG)-OH or tBoc-Ser(PEG)-OH is introduced into the LHRH analogue, and the produced PEG-LHRH analogue is recovered (without the protective group Fmoc or t-Boc).
JP Patent Application JP 3148298 describes PEG-peptide conjugates, e.g., PEG-GnRH conjugate, obtained by reacting the guanidino group of an arginine residue with PEG, while protecting the amino groups present in the molecules.
Citation of any document herein is not intended as an admission that such document is pertinent prior art, or considered material to the patentability of any claim of the present application. Any statement as to content or a date of any document is based on the information available to applicants at the time of filing and does not constitute an admission as to the correctness of such a statement.